This invention relates to a battery management system.
Battery-powered electronic devices, such as cellular phones, digital cameras, tablet terminals, portable music players, portable game apparatuses and notebook computers, all have built-in rechargeable batteries. Electronic circuits, such as CPUs (Central Processing Units) that control systems and process signals, liquid crystal panels, wireless communication modules and other analog or digital circuits, all receive their operating power from batteries.
FIG. 1 shows a block diagram of a battery-powered electronic device. An electronic device 500 comprises a battery 502 and a charging circuit 504 which charges the battery 502. The charging circuit 504 receives a source voltage VADP from an outer power adapter or a USB (Universal Serial Bus) and charges the battery 502.
A load 508 is connected to the battery 502. A battery current IBAT of the battery 502 is the difference between a charging current ICHG from the charging circuit 504 and a load current ILOAD of the load 508.
For battery-powered electronic devices, detection for remaining battery capacity (State Of Charge, SOC) is an essential function. In the electronic device 500, a remaining capacity detection circuit 506 is arranged. The remaining capacity detection circuit 506 is also called the fuel gauge IC (Integrated Circuit). There are two mainstream remaining battery capacity detecting methods using the remaining capacity detection circuit 506: (1) the voltage method, and (2) the coulomb count method (charge calculation method). The remaining capacity detection circuit 506 can also be built into the charging circuit 504.
The voltage method measures OCV (Open Circuit Voltage) of a battery in an open state (without any load), and estimates the remaining capacity according to the relation between OCV and SOC. OCV can only be measured when the battery is in an open state without any load. OCV cannot be correctly measured when the battery is under charging or discharging processes.
The coulomb count method measures the charging current flowing into a battery and the discharging current flowing out of the battery, and integrates the amount of charge being charged into and discharged from the battery, to estimate the remaining capacity. In contrast to the voltage method, the coulomb count method can estimate the remaining capacity while the battery is under operation, when the OCV cannot be measured.
The remaining capacity detection circuit 506 in FIG. 1 estimates the remaining capacity of the battery 502 using the coulomb count method. The remaining capacity detection circuit 506 includes a coulomb counter circuit 510 and an SOC calculator 512. The coulomb counter circuit 510 detects and integrates the battery current IBAT of the battery 502. A coulomb count value CC generated by the coulomb counter circuit 510 is expressed with the following equation:CC=∫IBATdt Strictly speaking, the battery current IBAT is sampled at discrete timings with a specified time period and is calculated by the following equation, where Δt is the sampling period:CC=Σ(Δt×IBAT)The integration can be performed on the assumption that a current IBAT flowing out of the battery 502 is a positive current and a current IBAT flowing into the battery 502 is a negative current.
The SOC calculator 512 uses the coulomb count value CC to calculate the SOC of the battery 502. The SOC is calculated using the following equation:SOC[%]=(CCFULL×CC)/CCFULL×100where CCFULL is the amount of charge (coulomb count value) stored in the battery 502 when the battery 502 is fully charged.